The performance of fiber-optics communication systems operating at high-speed are degraded significantly due to several transmission impairments including intrachannel and interchannel fiber nonlinearities, Gordon-Mollenauer effect and polarization-mode dispersion (PMD). The PMD is a quite challenging impairment to compensate due to its time variant and stochastic nature.
Orthogonal frequency division multiplexing OFDM is a special case of multi-carrier transmission in which a single information-bearing stream is transmitted over many lower rate sub-channels, and has already been used or proposed for a variety of applications including digital audio broadcasting, high-definition television (HDTV) broadcasting, high bit-rate digital subscriber line (DSL), IEEE 802.11, radio-over-fiber-based links, free-space optical communications, long-haul optical communications systems, multimode fiber links, and 100-Gb/s Ethernet. Due to orthogonality among sub-carriers in OFDM, partial overlap of neighboring frequency slots is allowed, thereby improving spectral efficiency as compared with a conventional multi-carrier system. Also, by using a sufficiently large number of sub-carriers and cyclic extension, the intersymbol interference (ISI) due to PMD can be reduced.
It has been shown that a low-density parity-check (LDPC)-coded turbo equalizer is able to successfully tackle the differential group delay (DGD) up to two bit-periods and residual chromatic dispersion over 700 km of SMF for reasonable trellis complexity of the Bahl-Cocke-Jelenik-Raviv (BCJR) equalizer. However, for DGDs above two-bit periods and longer SMF lengths the complexity of BCJR equalizer is prohibitively high for high-speed implementation, and an alternative approach is needed.
Accordingly, there is a need for simultaneous polarization-mode dispersion (PMD) and chromatic dispersion compensation for noncoherent fiber-optics communication systems that overcomes the limitations of current compensation techniques.